1. Field of the Invention
The present invention relates to a CMOS (complementary metal-oxide semiconductor) image sensor, and more particularly to a CMOS image sensor capable of preventing the degradation of spatial resolution when decreasing pixels for a color image, and generating a compressed image signal.
2. Description of the Related Art
A CCD (charge-coupled device) sensor is used as an image sensor in digital still cameras and video tape recorders. A CCD sensor differs from an ordinary CMOS structure, and in line with this, requires a fabrication process line with a degree of cleanness that is higher than the degree of cleanness for an ordinary CMOS LSI. Fabrication process line requirements such as this adversely affect the lowering of CCD sensor prices.
Meanwhile, attention is focusing on inexpensive versions of CMOS image sensors in place of CCD sensors. A CMOS image sensor can be fabricated via an ordinary CMOS fabrication process, since the CMOS image sensor constitutes a pixel using a photoelectric conversion circuit, which uses an MOS (metal-oxide semiconductor) transistor and a photodiode. As a result of this, a CMOS image sensor can be fabricated by a semiconductor fabrication line of an ordinary degree of cleanness, and, in addition, an image signal detecting circuit, and an image processing circuit comprising color processing can be formed on the same chip as an array of pixels, enabling a significant cost reduction compared to a CCD sensor. At present, the use of a CMOS image sensor as the image sensor of an inexpensive digital still camera has been proposed.
In a CMOS image sensor, RGB (red, green, blue) color filters are disposed on top of chip pixels, and pixels corresponding to each of these colors output detection signals (pixel signals) corresponding to the respective RGB gray scale values. Moreover, it is known that outputting more numerous detection signals of green (G), to which the human eye is most responsive, makes people highly sensitive, and an ordinary pixel array is a Bayer Space, which constitutes odd rows, which alternate red (R) and green (G), and even rows, which alternate blue (B) and green (G). In this array, green (G), which is the color for a luminance signal requiring high resolution, is arranged in a checkered pattern, and red (R) and green (G), which are the remaining color, are arranged in a checkered pattern therebetween.
As a result of this, a detection signal outputted from a pixel array is an RGB mosaic signal in the order of RGRG . . . GBGB . . . . This RGB mosaic signal is converted to an RGB gray scale signal (RGB simultaneous signal) for each pixel, and after a predetermined image processing, an image signal comprising an RGB gray scale signal corresponding to each pixel is outputted.
In an image sensor such as this, when a compressed image signal is outputted, the decreasing of a pixel signal is performed at a constant ratio. For example, when an image is displayed on a relatively small LCD (liquid crystal display) display panel provided on a digital still camera or the like, because the number of pixels of the LCD display panel are less than the number of pixels of the image sensor, decreasing processing (sub-sampling processing) becomes necessary.
A method carried out for a CCD sensor is decreasing processing, wherein fixed pixel data is decreased after the A/D conversion of a pixel signal outputted from a pixel array, and such a method is being pursued for a CMOS image sensor as well. Or, as another method, decreasing processing is also possible by simply decreasing an image signal after generating an RGB simultaneous signal and performing color processing.
FIG. 1 is a diagram illustrating conventional decreasing for an RGB mosaic signal. In the pixel array 10, RGRG . . . pixels are arranged in the odd numbered rows, and GBGB . . . pixels are arranged in the even numbered rows. The pixel signals of each row are outputted to a column line CL by driving a row selecting line ROW according to a vertical scanning circuit 12. The pixel signals outputted to a column line CL are held by a sample-and-hold circuit SH provided for each column, and are outputted from an output line 16 via column gates CG. Column gates CG are sequentially selected by column selection signals CS1-CS8 from a column selecting circuit 14, and pixel signals, which the sample-and-hold circuits SH of the respective columns are holding, are sequentially outputted.
Thus, pixel signals outputted from output line 16 are in the order of RGRG . . . GBGB . . . , and by decreasing every two pixels from these pixel signals, one-fourth of all the pixel signals are decreased. The pixels enmeshed inside pixel array 10 of FIG. 1 are pixels corresponding to pixel signals, which are read out by decreasing processing.
However, the above-mentioned decreasing processing method requires that hardware for decreasing processing beadded to the stage subsequent to an A/D converter. The addition of such hardware adversely affects CMOS image sensor cost-cutting.
Further, for the former decreasing processing, as shown in FIG. 1, an RGB mosaic signal is decreased at four pixels of two rows and two columns from among 16 pixels of four rows and four columns, and moreover, four pixels adjacent to a corner within the 16 pixels are outputted. A four-pixel RGB simultaneous signal is generated from the output RGGB pixel signals for these four pixels. As a result of this, the spatial resolution of a pixel signal subjected to decreasing processing deteriorates.